1. Field of the Invention
The present invention relates to a negatively chargeable toner for developing latent electrostatic images, and more particularly to a negatively chargeable toner for use in a one-component or two-component dry type developer which is utilized in the electrophotographic method, electrostatic recording method and electrostatic printing method.
2. Discussion of the Background
For developing latent electrostatic images to visible images with a developer, there are conventionally proposed two methods, that is, one using a two-component developer comprising a toner and a carrier, and the other using a one-component developer comprising a toner, as disclosed in Japanese Laid-Open Patent Application 61-147261.
According to the method using the two-component type developer, a carrier acquires a charge of one polarity and a toner acquires a charge of the opposite polarity by the friction therebetween while the carrier and the toner are mixed and stirred together. The toner is attracted to a latent electrostatic image charged to a polarity opposite to that of the toner and the latent electrostatic image can thus be developed with the toner. Development techniques are divided into several groups according to the kinds of toner and carrier, for example, the magnetic brush development using iron powder carrier, the cascade development using beads carrier, and the fur brush development.
The developing method using the one-component type developer includes the powder cloud development in which a toner is used in the form of a mist, namely, an extremely fine powder, the contact development (or touch-down development) by which toner particles are brought into direct contact with the surface of a latent-electrostatic-image-bearing member, and the induction development by which a magnetic electroconductive toner is brought into direct contact with the surface of a latent-electrostatic-image-bearing member.
The toner applicable to the above-mentioned various development processes comprises toner particles, each toner particle comprising a binder resin such as natural resin or synthetic resin, and a coloring agent such as carbon black dispersed in the binder resin. For instance, a toner can be prepared by dispersing a coloring agent in a binder resin such as polystyrene and pulverizing the above mixture until the particle diameter reaches about 1 to 30 .mu.m. Further, a magnetic toner can be obtained by addition of a magnetic material such as magnetite to the aforementioned components.
The toner used in previously mentioned developing methods is designed to acquire a positive or negative polarity in accordance with the polarity of the latent electrostatic images to be developed. The toner can be charged to a desired polarity by the triboelectric charging of a resin component contained in the toner. When the toner is charged by the triboelectric charging, however, the charge quantity of toner is relatively small, so that the obtained images become vague due to the fogging. Therefore, a dye, pigment or charge controlling agent capable of imparting the charging characteristic to the obtained toner is conventionally contained in the toner to successfully achieve the triboelectric charging of the toner.
Examples of the conventional charge controlling agent capable of negatively charging the toner include a metal complex salt of a monoazo dye; nitrohumic acid and salts thereof; metal complexes, such as Co-, Cr- and Fe-complexes of salicylic acid, naphthoic acid and dicarboxylic acid; sulfonated copper phthalocyanine pigment; a nitro-group- or halogen-introduced styrene oligomer; chlorinated paraffin; and melamine resin. The structures of the above charge controlling agents are complicated and the characteristics thereof are unstable. In addition, they are apt to decompose in the course of kneading under application of heat thereto, and they are easily caused to deteriorate by the application of mechanical shock and friction thereto, or by the changes in temperature and humidity. As a result, the charge controlling properties of the above-mentioned conventional charge controlling agents are degraded. In addition, the charge-imparting capabilities change depending upon the environmental conditions in most of the above conventional charge controlling agents. When the toner comprising the above-mentioned charge controlling agent is used in an electrophotographic image forming apparatus over a long period of time, there occurs the problem of toner-filming, which is associated with defective charging.
Recently, a binder resin comprising polyester resin or epoxy resin is widely used in the toner. This is because the above-mentioned binder resin has the advantages that the binder resin can be prevented from fusing and adhering to a vinyl chloride sheet, the original color of a coloring agent contained in the toner is not impaired, and both the storage stability and the image-fixing property at low temperature can be satisfied at the same time.
However, when the binder resin comprising polyester resin or epoxy resin is contained in the toner, the charge quantity of toner is insufficient at the initial stage, or decreased during repeated operations even though the initial charge quantity of toner is sufficient. Consequently, the fogging and the scattering of toner particles easily take place in the practical use. The above-mentioned defects are ascribed to the chemical structures of the polyester resin and the epoxy resin. More specifically, it is supposed that functional groups such as --COOH group and --OH group remain in the polyester resin and the epoxy resin, which hinder the maintenance of charging characteristics of the toner in a stable condition.
Further, the binder resin comprising polyester resin is superior in the image-fixing property at low temperature, although the grindability is poor in the preparation of the toner. A binder resin comprising a styrene-acryl-based resin is not satisfactory when used in the toner from the viewpoint of the image-fixing property at low temperature.
A binder resin for use in the toner disclosed in Japanese Laid-Open Patent Application 2-127657 comprises polyester resin, and other resins with a number-average molecular weight of 11,000 or less, such as styrene and a styrene-acryl-based resin. In this case, to improve the grindability of the polyester resin, the resins such as styrene and styrene-acryl-based resin are added to the polyester resin in an amount of up to 30 wt. % of the total weight of the binder resin. However, when the binder resin comprises the polyester resin and the styrene-acryl-based resin with a low molecular weight, the binder resin is apt to fuse and adhere to a vinyl chloride sheet, so that the vinyl chloride sheet is easily stained with the binder resin of the toner when brought into contact with an image-receiving medium carring the toner images thereon.